The invention relates to a method for stroboscopically examining repeating processes, in particular for examining moving vocal folds, and to an arrangement for performing said method.
The method of stroboscopy has been known for years. Various arrangements for observing/examining periodic processes of moving vocal folds are known, too. These arrangements include illumination units based on flash lamps or LEDs in combination with a CCD or CMOS camera system.
The flash lamps or LEDs are triggered by the fundamental frequency of the voice thus producing a stationary image of the vocal folds (see FIG. 1) or, if the phases of the trigger signal are continuously shifted (0°-360°), a slowly moving image of the vocal folds (see FIG. 2).
The disadvantage of these currently available examination arrangements based on stroboscopy instruments is the missing or imperfect synchronization or control of the illuminating flashes to the camera system. This disadvantage causes fluctuations of the brightness of the camera system that are perceived by the user as an unpleasant flickering.
DE 10 2008 015 500 A1 describes a method that shall avoid the different exposure of single images by interrupting the lightning impulse during the selection phase of the image sensor (no receipt of brightness information by the image sensor) and subsequently continuing the lightning impulse. However, this method does not produce identically exposed single images and thus it does not avoid the image flickering because the constancy of the light quantity per single image is not guaranteed. Moreover, the method according to DE 10 2008 015 500 A1 is only defined for flash intervals (corresponding to the fundamental frequency of the voice) that are longer than one partial period of an image sensor. For a 50 Hz camera system, this would be equivalent to a flash frequency of less than 50 Hz. The fundamental frequency of the human voice, however, ranges between 70 and 1000 Hz.
DE 699 18 460 T2 explains a method for avoiding brightness fluctuations in the video image in which a constant exposure of the single images is ensured by a constant number of lightning impulses of constant duration per single image. This method has the disadvantage that the constant quantity of light per single image is achieved by suppressing complete lightning impulses. Said disadvantage can produce dark or considerably post-amplified and thus “noisy” video images, in particular for low fundamental voice frequencies (few lightning impulses per single image).
Furthermore, the method used in DE 699 18 460 T2 requires that the exposure control of the camera is turned off because otherwise brightness fluctuations in the video image are produced due to the exposure control of the camera. If the light quantities per single image are high, an activated exposure control of the camera will block the receipt of brightness information by the image sensor for a specific period. If one lightning impulse comes exactly in this blocked period, it will be suppressed by the camera exposure control and thus it does not contribute to the brightness of the video image. This suppression is not detected in this method and therefore it will lead to differently exposed single images if the camera is turned on and consequently to brightness fluctuations in the video image.
However, a deactivation of the exposure control of the camera has the disadvantage that the brightness of the video image is not controlled any longer. Thus, crossfades will occur in the video image if very bright objects are viewed and dark objects will be difficult to see.
Moreover, DE 699 18 460 T2 reveals a standard generator circuit that generates a signal for providing the illumination within time intervals in which a trigger signal does not exist for the lightning impulses. According to DE 699 18 460 T2, this standard generator circuit is not synchronized with the camera system. Consequently, the flashes produced by this generator circuit will cause brightness fluctuations in the video image even if the camera exposure control is turned off. The video image will flicker if a trigger signal is not provided.